1. Field of the Invention
The present invention relates to an improvement in a semiconductor laser and method of making the same. Especially the present invention relates to a semiconductor laser made by an epitaxial growth method.
2. Prior Art
Hitherto, various proposals configurations and methods of making were made concerning semiconductor lasers for stable fundamental transverse mode operation. In general, fundamental transverse mode operation can be obtained by making the width of a stripe shaped active region narrow, thereby confining only a lowest mode light inthe narrow stripe shaped active region.
Two of the inventors of the present invention already proposed a novel improvement in semiconductor lasers in U.S. patent application Ser. No. 40,182 filed on May 18, 1979, now U.S. Pat. No. 4,296,387. The proposed improvement has the construction as shown in FIG. 1, wherein the laser device is made by multi-layer epitaxial growths on a semiconductor substrate 1. The method of making the semiconductor laser of FIG. 1 has the steps of
forming a terraced part with a specified step 1' on a principal face of a semiconductor substrate 1, PA1 epitaxially growing on the terrace shaped substrate 1 PA1 a first layer 2 as a first clad layer, having a thick part at the foot of the step 1', PA1 a second layer 3 as an active layer having an oblique part disposed on the part of the foot of said step 1', in a manner to have a pair of parallel bent parts between said oblique part 3' and upper and lower horizontal parts extending from the oblique part 3', PA1 a third layer 4 as a second clad layer, PA1 a fourth layer 5 as an ohmic contacting layer, PA1 forming a current limiting layer 6 of, for example an insulating layer with a stripe shaped opening 6', PA1 forming an ohmic electrode 7 on the current limiting layer 6 to contact the fourth layer 5 through the opening 6', and PA1 forming another ohmic electrode 8 on the bottom of the substrate 1,
The abovementioned first clad layer 2 is formed in a manner to have thick part 21 at the foot of the step 1' and thin parts on both sides of the thick part 21, so that at the thick part 21 the light does not easily leak out into the substrate 1 and at the thin parts the light easily leaks out into the substrate 1. The active layer 3 has a pair of parallel bent parts defining an oblique lasing region 3' inbetween, with the oblique lasing region 3' being thicker an upper horizontal part and a lower horizontal part of the active region 3. Accordingly, the active region has a construction of a rib type light guide, wherein the light is confined within the space defined between two parallel bent parts. By designing the width between the bent parts narrow to be able to confine only the fundamental transverse mode, a simple spot lasing was obtained.
In the abovementioned laser shown in FIG. 1, the current injected from the electrode 7 is liable to wider spreading on the lower part of the active region 3' and on the lower horizontal part of the active layer 3 than on the central or upper part of the active region 3' because of difference of the distance from the electrode 7. This means that the current injection efficiency of the device of FIG. 1 is not sufficient in comparison with an ideal state that the injected current passes only the active region. Therefore, in order to further improve the current efficiency, a measure to concentrate the current only to the oblique active region to prevent passing of current through the lower horizontal part of the active layer 3 would be considered desirable.